1. Field of Invention
This invention relates generally to compression and decompression systems and methods. More specifically, this invention relates to compression and decompression systems and methods that compress and decompress image areas containing edges of marks to be rendered in image data based on the direction of the edges of the marks.
2. Description of Related Art
The human viewer appreciates viewing non-continuous toneart information, e.g., text and/or lineart, at higher spatial resolutions than the spatial resolutions required for continuous toneart information, e.g., halftone information, because the human eye sees contrast information at a higher spatial resolution than color information. Therefore, more spatial resolution is necessary to render non-continuous tone regions than is necessary to render continuous tone regions. This differentiation between the amount of information necessary for the human eye to process non-continuous tone regions and to process continuous tone regions is due to hyperacuity. Hyperacuity is the human visual system's ability to differentiate locally misaligned edges of marks in a rendered image to a much finer extent than the receptor spacing of the human eye. It is not the frequency response, i.e., resolution, of the visual system, but the ability to reckon edge position with high precision that is most important.
Data transmitted in a bytemap, i.e., one byte for each pixel of the image, typically has a corresponding spatial resolution that is roughly equal to the size of the pixel. Forming high quality continuous tone regions does not require as much spatial resolution for the transmitted data. However, continuous tone regions require a high number of tone levels to minimize contouring, or the ability of the visual system to see changes in tone. In contrast, forming extremely detailed non-continuous tone marks, such as three or four point text, requires a significant amount of spatial resolution for the transmitted image data, but generally requires fewer number of tone levels.
In this case, the non-continuous tone pixels are not necessarily binary, but can also be composed of a number of gray levels. The non-continuous tone data is of the type that might be scanned in from a high quality scanner, or of a type called antialiased, which contains partial intensities to help in the removal of stairstepping or the positioning of edges.
Therefore, in a bytemap, high spatial resolution, e.g., 800×800 pixels per inch (ppi) is necessary for non-continuous tone regions, while continuous tone regions only need low spatial resolution, e.g., 400×400 (ppi). Therefore, transmitting bytemapped image data with high spatial resolution results in an unnecessary degree of spatial resolution for rendering continuous tone data and a waste of image system resources to process the unnecessary data.
If bitmaps are used instead to transmit image data for printing, continuous tone data is sent as prehalftoned dot shapes. Sending proper pre-halftoned dots to a destination, e.g., a printer, requires a high spatial resolution, e.g., 600×4800 ppi, to avoid contouring. However, 4800 pixels per inch resolution is too much resolution for non-continuous tone data.
Therefore, when using bitmaps, higher spatial resolution is necessary for rendering continuous tone data than is necessary for rendering non-continuous tone data, once again resulting in a waste of image system resources to process the unnecessary data.